Investigators in the electric motor arts have been called upon to significantly expand motor technology from its somewhat static status of many decades. Improved motor performance particularly has been called for in such technical venues as computer design and secondary motorized systems carried by vehicles, for example, in the automotive and aircraft fields. With progress in these fields, classically designed electric motors, for example, utilizing brush-based commutation, while relatively inexpensive, have been found to be unacceptable or, at best, marginal performers.
From the time of its early formation, the computer industry has employed brushless d.c. motors for its magnetic memory systems. The electric motors initially utilized for these drives were relatively expensive and incorporated a variety of refinements, for instance as necessitated with the introduction of rotating disc memory. Over the recent past, the computer industry has called for very low profile motors capable of performing in conjunction with very small disc systems and at substantially elevated speeds.
Petersen, in U.S. Pat. No. 4,745,345 entitled “D.C. Motor with Axially Disposed Working Flux Gap”, issued May 17, 1988, describes a PM d.c. motor of a brushless variety employing a rotor-stator pole architecture wherein the working flux gap is disposed “axially” with the transfer of flux being in parallel with the axis of rotation of the motor. This “axial” architecture further employs the use of field windings which are simply structured, being supported from stator pole core members, which, in turn, are mounted upon a magnetically permeable base. The windings positioned over the stator pole core members advantageously may be developed upon simple bobbins insertable over the upstanding pole core members. Such axial type motors have exhibited excellent dynamic performance and efficiency and, ideally, may be designed to assume very small and desirably variable configurations.
Petersen in U.S. Pat. No. 4,949,000, entitled “D.C. Motor”, issued Aug. 14, 1990 describes a d.c. motor for computer applications with an axial magnetic architecture wherein the axial forces which are induced by the permanent magnet based rotor are substantially eliminated through the employment of axially polarized rotor magnets in a shear form of flux transfer relationship with the core components of the stator poles. The dynamic tangentially directed vector force output (torque) of the resultant motor is highly regular or smooth lending such motor designs to numerous high level technological applications such as computer disc drives which require both design flexibility, volumetric efficiency, low audible noise, and a very smooth torque output.
Petersen et al, in U.S. Pat. No. 4,837,474 entitled “D.C. Motor”, issued Jun. 6, 1989, describes a brushless PM d.c. motor in which the permanent magnets thereof are provided as arcuate segments which rotate about a circular locus of core component defining pole assemblies. The paired permanent magnets are magnetized in a radial polar sense and interact without back iron in radial fashion with three core components of each pole assembly which include a centrally disposed core component extending within a channel between the magnet pairs and two adjacently inwardly and outwardly disposed core components also interacting with the permanent magnet radially disposed surface. With the arrangement, localized rotor balancing is achieved and, additionally, discrete or localized magnetic circuits are developed with respect to the association of each permanent magnet pair with the pole assembly.
Petersen in U.S. Pat. No. 5,659,217, issued Aug. 19, 1997 and entitled “Permanent Magnet D.C. Motor Having Radially-Disposed Working Flux-Gap” describes a PM d.c. brushless motor which is producible for incorporation into products intended for the consumer marketplace. These motors exhibit a highly desirable heat dissipation characteristic and provide improved torque output in consequence of a relatively high ratio of the radius from the motor axis to its working gap with respect to the corresponding radius to the motors' outer periphery. The torque performance is achieved with the design even though lower cost, or, lower energy product permanent magnets may be employed with the motors. See also: Petersen, U.S. Pat. No. 5,874,796, issued Feb. 23, 1999.
The above-discussed PM d.c. motors achieve their quite efficient and desirable performance in conjunction with a multiphase-based rotational control. This term “multiphase” is intended to mean at least three phases in conjunction with either a unipolar or bipolar stator coil excitation. Identification of these phases in conjunction with rotor position to derive a necessary controlling sequence of phase transitions traditionally has been carried out with two or more rotor position sensors in discretely different positions. Employment of such mutually spaced multiple sensors adds a considerable cost for an electronic phase commutation structure, inasmuch as utilization of practical integrated circuit packaging to include the sensors is precluded. Simple time domain-based multiphase switching has been considered to be unreliable and impractical since the rotation of the rotor varies in terms of speed under load as well as in consequence of a variety of environmental conditions and “sensorless” controllers which utilize back EMF sensing add considerable cost to the controller over those which use position sensors.
Petersen in U.S. Pat. No. 6,891,343, issued May 10, 2005 entitled “Multiphase Motors With Single Point Sensing Based Commutation” describes a simplified method and system for control of multiphase motors wherein a single sensor is employed with an associated sensible system to establish reliable and more cost effective phase commutation sequencing.
Over the years of development of what may be referred to as the Petersen motor technology, improved motor design flexibility has been realized. Designers of a broad variety of motor driven products including household implements and appliances, tools, pumps, fans and the like as well as more precise systems such as disc drives now are afforded an expanded configuration flexibility utilizing the new motor systems. No longer are such designers limited to the essentially “off-the-shelf” motor varieties as listed in the catalogues of motor manufacturers. Now, motor designs may become components of and compliment the product itself in an expanded systems design approach.
During the recent past, considerable interest has been manifested by motor designers in the utilization of magnetically “soft” processed ferromagnetic particles in conjunction with pressed powder technology as a substitute for the conventional laminar steel core components of motors. So structured, when utilized as a stator core component, the product can exhibit very low eddy current loss which represents a highly desirable feature, particularly as higher motor speeds and resultant core switching speeds are called for. As a further advantage, for example, in the control of cost, the pressed powder assemblies may be net shaped wherein many intermediate manufacturing steps and quality considerations are avoided. Also, tooling costs associated with this pressed powder fabrication are substantially lower as compared with the corresponding tooling required for typical laminated steel fabrication. The desirable net shaping pressing approach provides a resultant magnetic particle structure that is 3-dimensional magnetically (isotropic) and avoids the difficulties encountered in the somewhat two-dimensional magnetic structure world of laminations. See generally U.S. Pat. No. 5,874,796 (supra).
The high promise of pressed powder components for motors and generators initially was considered compromised by a characteristic of the material wherein it exhibits relatively low permeability. However, Petersen, in U.S. Pat. No. 6,441,530, issued Aug. 27, 2000 entitled “D.C. PM Motor With A Stator Core Assembly Formed Of Pressure Shaped Processed Ferromagnetic Particles”, describes an improved architecture for pressed powder formed stators which accommodates for the above-noted lower permeability characteristics by maximizing field coupling efficiencies.
Motor and generator technology has been advanced with respect to architectures exhibiting what has become to be known as “vertical stator” devices. As described in U.S. Pat. No. 6,617,747 by Petersen, entitled “PM Motor and Generator With a Vertical Stator Core Assembly Formed of Pressure Shaped Processed Ferromagnetic Materials”, issued Sep. 9, 2003 this architecture combines a radially directed magnetic flux transference at a working gap with a pressed powder-based stator structure wherein the stator poles are parallel with the device axis. Improvements in performance and expanded design latitudes are observed.
A radially aligned stator structure achieving enhanced performance with these pressed powder core materials is described in U.S. Pat. No. 6,441,530 by Petersen entitled “D.C. PM Motor With A Stator Core Assembly Formed Of Pressure Shaped Processed Ferromagnetic Particles” issued Aug. 27, 2002.
As the development of pressed powder stator structures for electrodynamic devices such as motors and generators has progressed, investigators have undertaken the design of larger, higher power systems. This necessarily has lead to a concomitant call for larger press molded configurations. The associated pressing process requires significant pressing pressures in order to evolve requite material densities to gain adequate electrical properties. To achieve those densities, press molding is needed in the 40 tons per square inch to 50 tons per square inch range. As a consequence the powder metal pressing industry requires that the molded parts exhibit aspect ratios (width or thickness to length in the direction of pressing) equal to or less than about 1:5 to maintain uniformity throughout the part. Thus as the lengths of stator core component structures increase, their thickness must increase to an extent that a resultant shape becomes so enlarged in widthwise cross section as to defeat important motor design goals, with attendant loss of both the economies of cost and enhanced performance associated with the emerging pressed powder technology.
Petersen, in application for U.S. patent Ser. No. 10/747,538 filed Dec. 29, 2003 entitled “Electrodynamic Apparatus and Method of Manufacture”, describes apparatus and manufacturing method for producing larger stator structures comprising pressed powder technologies. While retaining practical shapes and desirable dimensional characteristics, the stator structures are formed to exhibit requisite stator core particle densities. These advantageous aspects are achieved through the utilization of a modular approach to stator core formation.
Production costs for electronically commutated multi-phased motors can further be minimized by controlling the cost of involved electronic components, in particular, by limiting their number. For instance, the number of power devices or switching components required for multi-phased motors is minimized where a unipolar or single sided drive topology is employed. With typical unipolar configurations, the field windings of a given phase are energized in a singular polar sense, i.e., a north polar sense or a south polar sense. Thus, only one power device is required for each phase of the commutational scheme. However, at the point of commutation from one phase to a next in a commutational sequence, the energy necessarily stored in the phase being released must be accommodated for. Collapsing field induced back EMF or flyback voltage spikes encountered will exhibit amplitudes which are 20 times to 30 times that of the supply input to these unipolar configurations. Particularly as mechanical motor power is increased this calls for cost effective methods for removing the negative effects of the flyback voltage spikes. Heretofore, avoidance of those cost constraints has only been achieved through the utilization of unipolar systems with quite low power motor applications as may be evidenced with fan drives for computer applications.
In contrast to a unipolar structure approach, bipolar topology involves the excitation of field windings between one polar sense and an opposite polar sense. In terms of performance, the bipolar approach exhibits many advantageous aspects. For example, 3-phase unipolar systems are commutated at a theoretical 50% of maximum torque level, and, in effect, make use of one third of the total field winding array to achieve rotational drive. In comparison, considerably more expensive bipolar systems with twice the number of power switching devices commutate theoretically at 86% of maximum torque level and, in effect, utilize two thirds of the field winding assemblage to achieve a comparatively higher level of performance. Additionally, the bipolar architecture controls the flyback energy present in a collapsing field.
Notwithstanding these enhanced performance aspects associated with bipolar phase excitation design, the marketplace for electric motors is cost driven. Should practical electronic commutation approaches be achieved for higher power multiphase single sided drive systems, for example, in the 10 watt to 100 watt and above mechanical drive power range, the many advantageous performance and design flexibility aspects of brushless motor technology, albeit with unipolar based performance, will be made available to commerce at costs competitive with dated d.c. brush, universal and a.c. induction motor technologies.
Bringing the cost of electronically commutated unipolar multiphase motors to competitive and practical levels with the ubiquitous brush commutated motors, calls for corrective innovation with respect to three basic aspects, to wit; (a) overcoming the high costs associated with the use of multiple sensors in the commutation scheme; (b) making effective use of and practically forming pressed powder stator structures; and (c) providing a commutation system of competitive cost which practically and efficiently treats collapsing field back EMF or flyback phenomena. Thus, a trilogy of innovations are necessitated to bring the highly desirable features of multi-phase unipolar motor architecture into esse. One palliative component of that trilogy addressed to aspect (a) is the single sensor based commutation system described in application for U.S. patent Ser. No. 10/706,412 (supra), The second solutional component of that trilogy addressed to aspect (b) is the improved pressure shaped processed ferromagnetic particle stator technology described in U.S. Pat. Nos. 6,441,530; 6,617,747; and 6,707,224 (supra) and the modular approach to stator core formation described in application for U.S. patent Ser. No. 10/747,538 (supra).